This article presents both a proposal for a linear PID composite controller, composed of slow and fast sub-controllers, for flexible link robot systems modeled using the singular perturbation approach, and an efficient tuning method for the proposed controller structure. For the slow sub-controller, a PD controller with disturbance observer is used, which eventually takes on PID form. For the fast sub-controller, modal feedback PID control is utilized. The integral action in the controller removes steady state error in the joint caused by step disturbance and imperfect gravity compensation, although it also complicates the analysis. Effects of tuning the parameters of the controller to the closed loop response are investigated, and guidelines on the performance tuning for many flexible systems thus delivered. Through simulation and experiments, the adequacy and performance of the proposed method are verified.
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